I have a 1991 Hyundai excel and one of the pistons doesn't fire

Yes it does. A fuel injector is a device that atomizes and then delivers the fuel into the cylinder just before the piston squeezes the fuel and air mixture and the spark from the spark plug fires. Ergo your 2000 Hyundai Elantra being a 4 cylinder vehicle, has 4 fuel injectors.

Hi there sound very much like a failed fuel pump, they are electric and is possibly under your back seat, turn ignition on a listen closely for a humming noise, if not there then I would say you need to replaces the pump, check to see if you have power to the pump first, use a test light, hope this helps cheers good luck

It can be faulty or dirty spark plug itselfd.Also for reference get the fuel level checked.Is the fuel tank is empty or near to empty.
Try cleaning the spark plugs and then see if that is helpin.On some cases the black carbon layer gets collected on the spark plugs and that causes the problem,so cleaning the plugs help.But if its not helping,then spark plug has to be replaced.Spark plugs get power from ignition coil to ignite sparks.If ignitioncoils are faulty then also there will no spark.--------------

To troubleshot and confirm the issue,i suggest you to go through the help links provided below,check the procedure and it will help you to confirm the issue in your case.
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Click the link below:------

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http://howtobyme.blogspot.com/2011/12/car-crankshas-spark-but-will-not-start.html

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The spark plug firing order for car?

http://schematicsdiagram.blogspot.com/2011/06/spark-plug-firing-order-for-car.html

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How to Replace Spark Plugs?

http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-spark-plugs.html

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How to Perform a Basic Engine Tune Up-Spark Plug and Wire Change

http://schematicsdiagram.blogspot.com/2011/08/how-to-perform-basic-engine-tune-up_01.html

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http://schematicsdiagram.blogspot.com/2011/06/car-has-no-spark.html

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http://schematicsdiagram.blogspot.com/2011/12/how-does-spark-plug-wires-go-on.html

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http://schematicsdiagram.blogspot.in/2012/04/no-spark-on-2008-chevy-avalanche.html

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http://schematicsdiagram.blogspot.com/2011/12/how-does-spark-plug-wire-go-on.html

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http://whoisbyme.blogspot.in/2012/06/ford-plug-wires-not-coming-out.html

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http://schematicsdiagram.blogspot.com/2011/12/ford-explorer-has-no-spark.html

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Chevy impala spark plug firing order diagrams?

http://technoanswers.blogspot.in/2012/02/chevy-impala-spark-plug-firing-order.html

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How to gap spark plug on Chevy Impala?

http://technoanswers.blogspot.in/2012/02/how-to-gap-spark-plug-on-chevy-impala.html

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http://technoanswers.blogspot.in/2012/02/spark-plug-replacing-for-chevy-impala.html

Chevy s-10 blazer 4.3l Spark Plug Firing Order?

http://howtobyme.blogspot.com/2011/12/chevy-s-10-blazer-43l-spark-plug-firing.html

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http://schematicsdiagram.blogspot.com/2011/12/1982-dodge-rampage-spark-plug-firing.html

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2006 Suzuki Forenza Spark plug Firing Order Diagram?

http://schematicsdiagram.blogspot.com/2011/12/2006-suzuki-forenza-spark-plug-firing.html

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1983 Ford Ranger Spark plug firing order?

http://schematicsdiagram.blogspot.com/2011/12/1983-ford-ranger-spark-plug-firing.html

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Firing Order for 1991 Lexus ES 250?

http://schematicsdiagram.blogspot.com/2011/12/firing-order-for-1991-lexus-es-250.html

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Firing order for 1992 Chevrolet 5.0 engine?

http://schematicsdiagram.blogspot.com/2011/12/firing-order-for-1992-chevrolet-50.html

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How to identify number one spark plug on the distributor cap?

http://schematicsdiagram.blogspot.com/2011/12/how-to-identify-number-one-spark-plug.html

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How to remove rear spark plugs on 1991 Buick Regal?

http://howtobyme.blogspot.in/2012/02/how-to-remove-rear-spark-plugs-on-1991.html

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These details will help.

Thanks.

Most four cylinders are: 1-3-4-2

Here is the diagram you ask for. While it says it is for the 3.0L V6, it is the same for the 4.0L V6 if that's what you have...

You really need to find the source of the air leak !
It there is a leak it will cause all sorts of mayhem.
A good way to find a vacuum leak is to spray some engine start or WD40 around the engine but away from the air intake , If the engine speeds up when you spray thats where you leak is.
Hope this helps.

http://www.autozone.com/N,26200242/shopping/specsSelect.htm

You can try the firing order if that doesnt work let me know ill keep helping

did it start before you changed plugs?

the firing order is
1-6-5-4-3-2